My son is in third grade (public school in central VA), where in science class they are trying to teach him that LIGHT IS MATTER.

With the possible exception of the crazy quantum mechanics, I've never seen any source claim that light (or radiation) is matter. My understanding is that light is not matter practically by definition: matter is all things that are not radiation.

Certainly this would be true in the relatively simple science that 3rd graders are getting, wouldn't it?

(Granted, light is treated as a _particle_ to explain certain interactions with matter, like the photoelectric effect. Also granted that light/radiation/energy can be transmuted into matter, such as for example gamma rays spontaneously transmuting into electron/positron pairs and vice-versa).

Related question for the physicists on the board:

What is the definition of matter?

What THINGS in our universe, other than light, are not matter?

Can the _impulse_ of a photon be taken as evidence that it is matter?

Boris



PS

What really angers me is that they set up an experiment in the class to determine what things are matter and what are not. Using a glass of water, full to the brim, things were introduced into the water (rock, air, and light) to see if the glass would overflow (thus showing the thing had volume, a defining quality of matter I suppose, next to mass).

My son did the experiment, determined that shining light into the glass did not make it overflow, and concluded that light was not matter. He was told he must have made a mistake! What the &^$#!? (Other kids found that shining light into the glass DID make it overflow. Our future in science! Go figure...)

I agree kids should be taught critical thinking, but this goes a bit too far;-)


PPS

I'm not sure the water glass / light experiment would result in an overflow even if light WAS matter... surely light would not displace very much water, even if it did stay in the glass for longer than two femtoseconds. What's worse, enough light would heat the water - i.e. enough light would be absorbed - causing it to expand slightly and the glass to overflow, showing that light was matter. Because photons absorbed by water would add to the water's volume. (This is a rhetorical statement, folks. I'm playing the devil's advocate as best I can. That's a scientific method, is it not?)

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I would say that light is not matter, and if a glass of water overflows when you shine a light on it, then odds are you bumped the table. Also note that in that experiment, if light were matter, then the water should continue to overflow until the glass is empty of water and full of light. Somehow I doubt that happens.

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energy and matter are equivalent in some respects, of course, and if you shine a torch onto a black hole it gets more massive.

is that what they are trying to say?

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probably the other kids just threw the whole flashlight into the glass #-o

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That's exactly what I was thinking... #-o

I am just a dumb old hick from nowhere Missouri however, didnt Mr. Einstein say E=MC2. Isnt mass, matter and energy interchangable? If you do the equation backwards you still get the same result?

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You can trade money for food; it doesn't mean you can eat money. To be less flippant, yes, matter and energy can be converted back and forth, and even energy has mass. But I'd still consider them to be two different things.

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Isnt the whole point this - that they do have mass otherwise they would be undetectable?

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I think matter and energy, while certainly related, are two different things. If you considered light to be bundles of photons, then (although massless) I think you can consider it matter. Though something tells me this is a bit sophisticated for third graders.

There are a lot of theroies attempting to explain what 'light' or photons are.

The problem is they usually either favor light being a particle alone, or light being a energy wave alone. When it's already been proven to exhibit the properties of both.

I cannot remember the source of this, but one of the theroies is that a photon is a combination of a +1/3 mass particle and a -1/3 mass particle. (where as an electron would be a +1 mass particle). It starts diving into what I call as a layman, mass/anti mass? particle interaction. The net effect of the two particles is a rest mass of zero, but as each particle is an opposite, they repel then atract, collide and convert to energy, which imediately cools down back into similar particles. They travel in the direction that the intial release of energy was going when it cooled to form the photon.

I'm not cetrain but I think this one is really saying that light is the only know case of two stable virtual particles.

It's still a theroy, but the only one I remember that explained how it might be both a particle, and an energy wave, and maintain itself without a net loss of speed or material. Basically a case of cyclic entropy. It's energy, cools to matter, collides, producing same amount of energy, and keeps doing this until it is absordbed by real matter (usually resulting in a shift of the electrons orbits)

Anyone know more about this theroy? I read about it over two years ago, and only understood the basics. But it did make a lot of sense.

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At the same time, aren't we doing them a disservice by giving them the wrong impression? Sure, you can tell the kids that light is energy, but teaching them that light is definitly matter at this early age is going to cause some confusion later on. Something tells me that this is just a confused third grade teacher. No offense to teachers, but I doubt an elementary ed major in college took enough physics to understand Einstein.

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I agree with you, though at the same time I'm not sure the students will absorb it enough to really have it make a difference for any future physicists.

Well, mass and matter are not interchangable, especially in Einstein's equations. And light, to my knowledge, has zero rest mass.

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Yeah, the energy of light comes from E^2=(pc)^2+(moc^2)^2. Since m is 0, E=pc, where p is the momentum.

Although I think the question of whether light is matter is a good one, and I'd still like to have an authoritative answer (please!), my point with the teacher (and possibly the school, and maybe the school system or even the state!) is that they've set up this experiment to show that light is matter, and then they ignore the experimental results. Now granted the experiment may be flawed (see my PPS in the initial post), the fact is my son conducted the experiment correctly, found a negative result, and now the teacher is telling him he's done it wrong.

THAT is the problem. No, he hasn't done it wrong. Just because the result is not what the teacher expected, does not make it wrong. But before I get much farther into this with the "authorities," I thought I'd better check here to make absolutely sure I haven't missed anything.

I've already corresponded with the teacher, to ask her what source(s) indicate to her that light is matter. Depending on her reply (not yet received) I may have to take this further up the educational command chain.

Again, I agree that teaching the FACT that light is matter is not such a big problem. They will learn the "truth" sooner or later, whatever it may be. But if it is a FACT that light is NOT matter, and if a student's work has shown that light is not matter, then I do think it is a big problem if the teacher asks the kid to sweep his results under the table just so the teacher's scientific cart is not upended. This the exact opposite of what kids should learn about science.

(Thanks to all this they may soon learn how much controversy science research can generate!)

If I may assume that light is NOT matter, then the situation right now in this class is that shoddy experimenting (or worse: fraud) is being rewarded by this teacher, and careful experimenting is being punished.

So yes, I claim this is a crucial issue for our future physicists.

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This is interesting to me. Does this explain why a photon can exert a force (on say a solar sail) even though it has no mass? How can something without mass have momentum? (We know that classical, newtonian momentum p = mv) It's got to have something to do with relativity I guess.

This relates to my question (#3 in the initial post), can the impulse of a photon be taken as evidence that the photon is matter.

Boris

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I appreciate everyone's contributions so far - they've not yet shaken my understanding of what light and matter are, and that they are different!

But I notice no one has tried to address these three questions that I posted originally, which I consider crucial to this topic. I'll offer my own answers, by way of example, and maybe an expert can give me some support or shoot me down.

1. matter is all things that have mass and take up space (volume)

2. one thing other than matter in the universe might be a magnetic field, easily demonstrated to third graders. Thus "fields" or "forces" are something tangible, yet not matter.

3. the impulse of a photon does not make it matter see def. #1. Just because light may mediate a transfer of energy between two separate objects in space, does not make it matter. Magnetic fields do the same thing, and they are not matter either.

Any other ideas?

Boris

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Now that I've offered up a definition for matter, I can't help trying to define light... and I've had some strange thoughts!

Matter on the one side is clear to me, easily defined.

Forces or fields on the other side are more mysterious, but maybe can be defined generally as interactions between discrete matter particles. (these interactions can be described using particle exchanges, or distortions in space, etc.).

This sort of gives us a definition for all things not matter, except... light.

Light seems to be something in between the two. Light seems unique. (I should say electromagnetic radiation generally. Light is just a tiny subset of all electromagnetic radiation). On one hand light can be interpreted as a disturbance in the "non-matter" electromagnetic field, on the other hand it is seen to behave like a particle, nominally like matter, a photon.

So is light a third category of entity? Not matter, nor a field?

Boris

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You're so close. Light *is* all electromagnetic radiation. Electric and magnetic fields are produced/governed (I can't think of better words right now) by exchanges of photons. Every force (e-m, gravity, strong and weak nuclear) is governed by a force-transmitting particle (photon, graviton, Z & W bosons, IIRC). Light, a photon, falls right into where you want to put it. It is a particle that gives us e-m forces.

Time? Distance?

[Edit-1] What about the 4 states of matter (solid, liquid, gas, plasma)? If something's not one of those four things, can we say it's not matter?[/Edit-1]

I think the teacher needs to be clear about whether they are talking about rest mass or relativistic mass, but apparently "relativistic mass"is old fashioned now anyway. Have a look at what the Usenet Physics FAQ has to say about mass and light.

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I'm guessing that some students got the water to overflow because it came out of the tap cold, and expanded as it slowly warmed up to room temperature. If the light was bright that could have warmed it a bit too.

Please excuse me if I'm repeating someone elses answer. I just skimmed through the answers and didn't see this.

The obvious answer to the question: Is light matter? The answer can only be no. If light was matter it would have mass. If it had mass it could not travel at the speed of light because it would take more energy than exists in the universe to get it up to C.

It is a phase state of matter by virtue of E=mC^2 but that does not mean it is matter any more than ice is steam.

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I was trying to be clear that this topic is about electromagnetic radiation / photons generally. Because some readers here might consider light to mean just _visible light_, I wanted to clarify that I am using the word to mean the entire spectrum of e-m radiation, i.e. e-m radiation generally, or photons.




Now this I don't understand. You are treating light as a field?

So you mean when some electron transitions happen in the tungsten of a light bulb filament, the e-m field is disturbed (mediated by photon), and this disturbance ultimately makes it's way to some electrons in my retina, where they are affected by the disturbance, exciting a cell, signalling my brain, etc....? I've never thought of light that way, but I guess it could work! (It sounds a lot like the aether).

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Gotcha. It is hard to tell sometimes. Plus, I'm still getting blank stares from students when I tell them that radio waves travel at the speed of light, over 6 weeks into the semester.


No, I'm treating light as a particle (which it is). Although it looks like I said "fields" when I was thinking "forces." The fields do influcence the forces, but photons are the particles that transmit the force. (There's the word I should have used.)


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That sounds like a decent explanation, though I'm honestly not sure how good it is. Though, in light of my misspeaking (miswriting?) earlier, it might be better to say that the tungsten atom exerts a slight electromagnetic force on your retina, transmitted via the emitted photon. (Wow, that sounds better now than when I began to write it. )

Hmm, speed of light. It is only constant in a vacuum. Where exactly is a vacuum to be found? Interstellar space is thought to contain around 100,000 to 1,000,000 hydrogen atoms per cubic meter. A very thin and transparent gas to be sure, but not a vacuum. Also, the speed of light through transparent matter is frequency dependent, hence the prism and the rainbow. The speed of radio waves through a coaxial cable is around 1/2 C. The denser the medium the slower it is. Then we have a couple of exceptions. The Casimir effect.

Then there is quantum tunneling.

"According to Nimtz, Mozart's 40th Symphony hopped across 12 centimeters of space at 4.7 times the speed of light. What's more, Nimtz actually had a recording to prove it. To his now bemused audience, he played a tape in which among the background hiss strains of Mozart could be heard. This was the 'signal' that had traveled faster than light."

http://www.wsws.org/public_html/prio...b9-9/light.htm

The equivalence of matter and energy is not really debated. It is more a matter of semantics. There is interesting reading on this here:

http://plato.stanford.edu/entries/equivME/

Am I missing some conclusion here, or are you just throwing out more bits to speculate on?

I'd say it's wavelength dependent. The frequency stays the same (in most cases) while the wavelength changes.

A photon always travels at c. If they are travelling through a material they may appear to travel slower because they will get absorbed and re-emitted by atoms of the material. They travel at c between the emission/absorbtions.
Between the atoms of every material

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Frequency determines wavelength. Either is valid to express peak to peak time or distance of an electromagnetic oscillation.

Frequency does not stay the same as wavelength changes, it is the reciprocal of wavelength.

Worzel said "Between the atoms of every material"

The Casimir effect indicates otherwise. It is not conjecture either, it is an experimentally verfiable effect.

Just google'd "Casimir Effect". Interesting, but from what I read it appeared to be about a force between to surfaces in a vacuum.

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There are 10 types of people in the world. Those who understand ternary, those who don't, and those waiting for a bus.
If logic doesn't work, then surely it does.